(a) Field of the Invention
This invention relates to a manufacturing process of chlorine, and more particularly to an improvement for the process of producing chlorine by oxidizing hydrogen chloride gas with an oxygen containing gas.
(b) Description of the Prior Art
Chlorine is produced in large amounts by the electrolysis of sodium chloride. The electrolysis supplies a chlorine demand which has recently been in a sharp increase. The electrolysis of sodium chloride, however, co-produces sodium hydroxide which has a smaller demand than that of chlorine. Therefore it is difficult to successfully adjust the supply-demand imbalance, though the demand of chlorine and sodium hydroxide is tried to be met by the electrolysis of sodium chloride.
On the other hand, a great quantity of hydrogen chloride is produced as a by-product in the chlorination or phosgenation reaction of organic compounds. The amount of by-produced hydrogen chloride is far greater than the market demand of hydrochloric acid. Thus a large quantity of hydrogen chloride is discarded without utilization and moreover considerable expenses are required for its disposal.
Therefore, if chlorine can be efficiently recovered from by-produced hydrogen chloride which is abandoned as aforesaid in a large amount, the supply-demand balance of chlorine and sodium hydroxide will easily be able to be maintained by the conbination of sodium chloride electrolysis and oxidation of the by-produced hydrogen chloride.
The reaction of oxidizing hydrogen chloride to produce chlorine has been known as Deacon reaction for many years. In the reaction of preparing chlorine by oxidizing hydrogen chloride, the so-called "Deacon catalyst" which is a copper-base catalyst invented in 1868 has conventionally been considered to exhibit the highest activity. Since then, it has been proposed a number of catalysts which are added with various compounds as a third component to copper chloride and potassium chloride. The reaction temperatures, however, are required to be 450.degree. C.and above in order to oxidize hydrogen chloride at a practical applicable reaction rate by use of these catalysts. The high temperature causes problems such as reduction of catalyst life accompanied by the vaporizing of catalyst ingredients. The use of catalysts other than copper-base including, for example, iron base etc. have been proposed in order to eliminate these problems, and yet no catalysts have been known to exhibit satisfactory performance for practical application. For example, chromic-oxide-base catalysts have been suggested to be generally superior in high-temperature stability and lifetime to the copper-base catalysts. No results, however, have been reported to show enough activity. That is, U.K. Pat. No. 584,790 discloses an intermittent process. In this process, hydrogen chloride is introduced to form chlorine at about 400.degree. C. on the catalyst obtained by impregnating a suitable carrier with an aqueous solution of chromic acid or chromium nitrate and subjecting the impregnated carrier to thermal decomposition. When the catalyst is inactivated, the feed of hydrogen chloride is stopped and the catalyst is regenerated by introducing the air. Then the air flow is stopped and the feed of hydrogen chloride is started again to prepare chlorine.
Besides U.K. Pat. No. 676,667 discloses another process using a carrier-supported catalyst prepared from bichromate or dark-green chromic oxide, that is, unglowed chromia. In this process, hydrogen chloride and the oxygen containing gas are fed to react at a reaction temperature of 420 to 430.degree. C. Hydrogen chloride is obtained with a conversion ratio of 67.4% of the equilibrium value at a space velocitY of 380 Hr.sup.-1 and 63% at 680 Hr.sup.-1. When the reaction temperature is lowered to 340.degree. C., the reaction can also proceed and yet the conversion ratio obtained is only 52% even by maintaining the space velocity at a low level such as 65 Hr.sup.-1. The prior art further discloses that chromia can not necessarily be active as the oxidizing catalyst of hydrogen chloride. That is, amorphous chromia is active for the oxidation of hydrogen chloride. It is required to carry out the heat-treatment of chromic anhydride at a temperature of 400.degree. C. or less in order to prepare the amorphous chromia catalyst. Chromia is clearly illustrated to crystallize by heating above 500.degree. C. and to loose its catalitic activity for the hydrogen chloride oxidation.
Furthermore, the chromia catalyst has a short life in the oxidation of hydrogen chloride and cannot be employed for industrial operation. As a means of overcoming the drawback, U.K. Pat. No. 846,852 (corresponding to U.S. Pat. No. 3,006,732) discloses that the catalyst life can be extended by incorporating a small amount of chromyl chloride (CrO.sub.2 Cl.sub.2) in the raw material. As shown above, the intact chromia catalyst cannot be employed in continuous operation for a long period as a result of its short life. In addition, the patent also discloses that high activity is exhibited by the amorphous chromia prepared by calcining ammonium bichromate or chromic anhydride at a temperature of 500.degree. C. and less, preferably 350 to 400.degree. C.
As aforementioned, the conventionally known processes could not improve the drawbacks of short catalyst life, high reaction temperature and low space velocity, even though chromic oxide is used as the catalyst. Therefore these processes could not withstand the industrial operation unless new reacting agents, such as chromyl chloride were incorporated. That is, the traditional chromic oxide catalyst did not specifically exhibit excellent properties as compared with the copper-base catalyst.